[3H] Overflow evoked by 5 min supramaximal preganglionic stimulation at 1 pps has been studied in isolated guinea pig superior cervical ganglion preparations preincubated with [3H]choline. At 15 μM norepinephrine (NE) reduced both the [3H]choline overflow and endogenous acetylcholine release by 59.4 and 54.1% respectively; the dose-response curve for NE inhibitory action is described. Evidence is given that endogenous catecholamines effectively reduce ACh release from the ganglia. After blocking the inhibitory action of endogenous NE, a significant β-adrenoceptor-mediated facilitatory effect on ACh release could be observed. Preincubation of the ganglia with different combinations of α1 and α2 agonists (phenylephrine, 10 μM and clonidine, 1 μM respectively) and antagonists (prazosin, 10 μM and yohimbine, 3 μM) showed that the adrenoceptors involved in α-mediated NE inhibition of ACh output are exclusively of the α2-type.
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